The level of certain analytes in blood and other body fluids is often used to diagnose disease, determine disease risk factors, monitor the course of a therapy, or determine the presence of illicit drugs. In recent years, analytes carried in blood have been evaluated to determine various cholesterol and triglyceride levels as a significant indicator of risk of coronary heart disease. In managing heart disease, physicians commonly order what is referred to in the art as a “full lipid panel” for patients to determine the concentration of total cholesterol, high density lipoprotein cholesterol (HDL), low density lipoprotein cholesterol LDL), and triglycerides. Glucose and ketone dry test strips are used for managing diabetes. Ketone test strips also are useful in managing weight loss. Test strips for determining creatinine concentration in the blood or other bodily fluids are used for diagnosing and treating impaired kidney function and a variety of other metabolic disorders and diseases.
While clinical tests have been used and still are being used to determine the concentration of the above-mentioned analytes, more and more physicians and consumers are relying on dry test strips for economical and easier measurement, particularly when testing at shorter intervals, such as days or weeks, is important or when rapid results are critical.
The dry test strip assembly includes a dry test strip carrier and a fluid permeable strip. The dry test strip carrier generally is made of a plastic having a tensile strength of about 4,800 pounds per square inch (psi). The permeable strip includes several layers of material to separate the blood components, react the blood plasma with a particular reagent or reagents, and obtain a signal indicative of the concentration of the analyte. See, for example, U.S. Pat. No. 5,104,619 issued Apr. 14, 1992 to de Castro et al. and entitled “Disposable Diagnostic System”; U.S. Pat. No. 5,166,051 issued Nov. 24, 1992 to Killen et al. and entitled “Membranes, Membrane Overlays, For Exclusion of Erythrocytes, And Method Of Immunoassay of Whole Blood Analytes”; U.S. Pat. No. 4,774,192 issued to Terminello et al. on Sep. 27, 1988 and entitled “A Dry Reagent Delivery System With Membrane Having Porosity Gradient”; and U.S. Pat. No. 4,477,575 issued to Vogel et al. on Oct. 16, 1984 and entitled “Process And Composition For Separating Plasma Or Serum From Whole Blood”. In more recent systems, the dry test strip in the strip carrier is placed within a spectrophotometric device that evaluates the colorimetric response and assigns a quantitative value indicative of the analyte concentration in the blood, urine, or other bodily fluid sample. For example, see U.S. Pat. No. 5,597,532 issued to James Connolly on Jan. 28, 1997, owned by the assignee of the present invention, and entitled “Apparatus For Determining Substances Contained In A Body Fluid”, which patent is incorporated by reference to the same extent as though fully disclosed herein. Most modern dry test strip assemblies follow the teachings of the Connolly patent, utilizing a carrier comprising a base and a cover, with the fluid permeable strip held between the base and cover.
All of the above systems depend on the flow of the bodily fluid, i.e., blood, through the system as the driving force to separate the unwanted components from the components to be tested. The red blood cells are separated from the serum, and the analyte to be tested is isolated from the other components: in a system for testing HDL, for example, the lipoproteins other than HDL are isolated from the HDL. Thus, the control of the flow is important for the tests to be accurate. For example, U.S. Pat. No. 4,774,192 relies on a highly porous bottom layer to allow the fluid to flow easily and a dense upper layer to trap the unwanted components. In U.S. Pat. No. 4,477,575, a lateral flow of blood through a fiberglass layer is used to separate the components. U.S. Pat. No. 5,597,532 uses a vertical flow downward through membranes and a lateral flow outward in a lower membrane that is designed to absorb a large amount of fluid to drive the fluid flow. A rectangular test membrane that is significantly larger than the area of the circular opening through which a spectrophotometer reads the strip enhances this feature to encourage flow and prevent blood pooling in the test area of the membrane. These flow properties are determined by the permeable materials from which the strip material is made and by the carrier for the strip. If the strip is held loosely in the carrier, flow is augmented, but the strip can move, which can lead to erroneous results. If the strip is held firmly, damage can result, which leads to erratic results as well as inaccuracies. Thus, test strip carriers have been designed that permit vertical and lateral flow through most of the strip, but tightly hold other parts of the strip. See, for example, U.S. Pat. No. 5,597,532 referenced above. However, the accuracies of test strips still remain lower than similar tests performed in the laboratory.
The design of dry test strips and carriers also is constrained by the need to manufacture the strip. The strip and carrier should be able to be manufactured and assembled quickly but without negatively affecting the reliability and accuracy of the strip. Up until now, final automated systems used up several times the amount of fluid permeable strip than was required for the strips themselves, which was costly. To efficiently use the strip, the final assembly was by hand. This hand labor adds to the cost of the system.
It would be highly desirable to have a test strip system that improved the accuracy of the tests, but at the same time could be more economically manufactured, particularly one in which the hand labor was minimized without excessive use of strip material and without compromising the reliability and accuracy of the system.